Digital cameras are becoming very popular these days. Equipped with a solid state imaging device, such as a CCD (charge coupled device) image sensor or a CMOS (complementary metal oxide semiconductor) image sensor, the digital camera converts a subject image formed on a light receiving surface of the image sensor into digital image data, and then stores this data. Similar to the film cameras, some of the digital cameras have an aperture stop to limit the amount of incident light.
The CCD image sensor, for example, has a light shielding film on a silicon substrate, and this light shielding film has a matrix of openings. Formed inside of each opening is a photodiode that converts incident light into an electric charge and stores it. To increase the amount of light incident on the photodiodes, there are provided an inner-layer lens and a micro-lens on the photodiode in each opening. However, as shown in FIG. 10, the photodiodes have a light-receiving characteristic in which an amount of light received reaches a maximum when an angle of incidence is 0° (when the light vertically strikes the light receiving surface), and as the angle of incidence becomes wider, the amount of light received decreases (see, for example, Japanese Patent Laid-open Publication No. 2005-142429).
Due to this light-receiving characteristic, the amount of light received on each photodiode is not increased in the conventional digital cameras as much as expected from a photographic film in a film camera, even if the aperture stop is opened to increase the amount of incident light. This is because of the fact that what is increased by opening the aperture stop is oblique light which has little impact on light-receiving efficiency. As a result, an opening/closing effect of the aperture stop (variability in the amount of light received by opening and closing the aperture stop) is not enhanced, and aperture stop value change is not effectively reflected in a captured image.